The present invention relates to capacitive pressure transducers.
FIG. 1A shows a partially sectional side view of an assembled prior art capacitive pressure transducer assembly 100. FIG. 1B shows an exploded sectional side view of transducer assembly 100. For convenience of illustration, FIGS. 1A and 1B, as well as other figures in the present disclosure, are not drawn to scale. U.S. Pat. No. 4,823,603 discloses a capacitive pressure transducer assembly of the general form of transducer assembly 100.
Briefly, transducer assembly 100 includes a body that defines a first sealed interior chamber 110, and a second sealed interior chamber 112. Chambers 110 and 112 are isolated from one another by a relatively thin, flexible, conductive diaphragm 120. As will be discussed in greater detail below, diaphragm 120 is mounted so that it flexes, or deflects, in response to pressure differentials in chambers 110 and 112. Transducer assembly 100 provides a parameter that is indicative of the amount of diaphragm flexure and this parameter is therefore indirectly indicative of the differential pressure. The parameter provided by transducer assembly 100 indicative of the differential pressure is the electrical capacitance between diaphragm 120 and an electrode 130.
Transducer assembly 100 includes a P_x cover 140 and a P_x body 150 (as will be discussed below, the term xe2x80x9cP_xxe2x80x9d refers to an unknown pressure). FIG. 2A shows a top view of P_x body 150. P_x body 150 has a tubular shape and defines a central interior aperture 152 (shown in FIG. 2A and indicated by lines 153 in FIG. 1B). The upper surface of P_x body 150 is stepped and provides a shoulder 154 that extends around the perimeter of aperture 152. P_x body 150 also includes a lower surface 156. P_x cover 140 is a circular metallic sheet and is provided with a pressure tube 142 that defines a central aperture 144. P_x cover 140 is rigidly affixed to the lower surface 156 of P_x body 150 (e.g., by welding). Diaphragm 120 is normally a thin, circular, flexible sheet of conductive material (e.g., stainless steel). As stated above, FIGS. 1A and 1B are not drawn to scale, and diaphragm 120 is normally much thinner than illustrated in comparison to the other components of transducer assembly 100. Diaphragm 120 contacts shoulder 154 of P_x body 150 as indicated in FIG. 1A. The outer perimeter of diaphragm 120 is normally welded to P_x body 150 to rigidly hold the outer perimeter of diaphragm 120 to shoulder 154 of P_x body 150.
P_x cover 140, P_x body 150, and diaphragm 120 cooperate to define interior sealed chamber 110. P_x cover 140 defines the bottom, P_x body 150 defines the sidewalls, and diaphragm 120 defines the top of chamber 110. Fluid in tube 142 may flow through aperture 144, and through central aperture 152 (shown in FIG. 2A) into chamber 110. So, fluid in tube 142 is in fluid communication with the lower surface of diaphragm 120.
Transducer assembly 100 also includes a P_r body 160 and a P_r cover 170 (as will be discussed below, the term xe2x80x9cP_rxe2x80x9d refers to a reference pressure). FIG. 2B shows a top view of P_r body 160. P_r body 160 has a tubular shape and defines a central aperture 162 (shown in FIG. 2B and indicated by lines 263 in FIG. 1B). The upper surface of P_r body 160 is stepped and provides a lower shoulder 164 and an upper shoulder 166. Lower shoulder 164 extends around the perimeter of aperture 162, and upper shoulder 166 extends around the perimeter of lower shoulder 164. P_r body 160 also includes a lower surface 168 opposite to shoulders 164, 166. Lower surface 168 of P_r body 160 is rigidly affixed to the upper surface of the outer perimeter of diaphragm 120 (e.g., by welding). P_r cover 170 is a circular metallic sheet and is provided with a pressure tube 172 which defines a central aperture 174. P_r cover 170 is rigidly affixed to P_r body 160 (e.g., by welding) so that the outer perimeter of P_r cover 170 is in contact with upper shoulder 166 of P_r body 160.
P_r cover 170, P_r body 160, and diaphragm 120 cooperate to define interior sealed chamber 112. Diaphragm 120 defines the bottom, P_r body 160 defines the sidewalls, and P_r cover 170 defines the top of chamber 112. Fluid in tube 172 may flow through aperture 174, and through central aperture 162 (shown in FIG. 2B) into chamber 112. So, fluid in tube 172 is in fluid communication with the upper surface of diaphragm 120. As will be discussed below, electrode 130 is housed in, and does not interfere with the fluid flow in, chamber 112.
Electrode 130 is normally fabricated from a non-conducting (or insulating) ceramic block and has a cylindrical shape. FIG. 2C shows a bottom view of electrode 130. The lower surface of electrode 130 is stepped and includes a central face 135 and a shoulder 136 that extends around the outer perimeter of central face 135. Electrode 130 also defines an aperture 132 (shown in FIG. 2C and indicated by lines 133 in FIG. 1B). Electrode 130 further includes a relatively thin conductor 134 that is deposited (e.g., by electroplating) onto central face 135. Conductor 134 is explicitly shown in FIGS. 1B and 2C, and for convenience of illustration, conductor 134 is not shown in FIG. 1A. Electrode 130 is clamped between P_r cover 170 and lower shoulder 164 of P_r body 160 as shown in FIG. 1A. Aperture 132 (shown in FIG. 2C) in electrode 130 permits fluid to freely flow through electrode 130 between the upper surface of diaphragm 120 and pressure tube 172. Clamping electrode 130 to P_r body 160 holds conductor 134 in spaced relation to diaphragm 120. Electrode 130 is normally positioned so that the space between conductor 134 and diaphragm 120 is relatively small (e.g., on the order of 0.0002 meters).
Conductor 134 and diaphragm 120 form parallel plates of a capacitor 138. As is well known, C=Ae/d, where C is the capacitance between two parallel plates, A is the common area between the plates, e is a constant based on the material between the plates (e=1 for vacuum), and d is the distance between the plates. So, the capacitance provided by capacitor 138 is a function of the distance between diaphragm 120 and conductor 134. As diaphragm 120 flexes up and down, in response to changes in the pressure differential between chambers 110 and 112, the capacitance provided by capacitor 138 also changes. Because electrode 130 (and conductor 134) preferably remains stationary relative to the housing, electrode 130 may be referred to as the xe2x80x9creference electrode.xe2x80x9d At any instant in time, the capacitance provided by capacitor 138 is indicative of the instantaneous differential pressure between chambers 110 and 112. Known electrical circuits (e.g., a xe2x80x9ctankxe2x80x9d circuit characterized by a resonant frequency that is a function of the capacitance provided by capacitor 138) may be used to measure the capacitance provided by capacitor 138 and to provide an electrical signal representative of the differential pressure.
Transducer assembly 100 includes an electrically conductive feedthrough 180 to permit measurement of the capacitance provided by capacitor 138. One end 182 of feedthrough 180 contacts electrode 130. Feedthrough 180 extends through an aperture in P_r cover 170 so that the other end 184 of feedthrough 180 is external to transducer assembly 100. The aperture in P_r cover 170 through which feedthrough 180 extends is sealed, for example by a melted glass plug 185, to maintain the pressure in chamber 112 and to electrically insulate feedthrough 180 from P_r cover 170. Feedthrough 180 is electrically connected to conductor 134. Electrode 130 normally includes an electroplated through hole (not shown) to permit electrical connection between conductor 134 (on the bottom surface of electrode 130) and end 182 of feedthrough 180 which contacts the top surface of electrode 130. So, feedthrough 180 provides electrical connection to one plate of capacitor 138 (i.e., conductor 134). Since diaphragm 120 is welded to P_r body 160, P_r body 160 provides electrical connection to the other plate of capacitor 138 (i.e., diaphragm 120). So, the capacitance provided by capacitor 138 may be measured by electrically connecting a measuring circuit (not shown) between P_r body 160 and end 184 of feedthrough 180. In practice, the body of transducer assembly 100 is normally grounded, so the capacitance provided by capacitor 138 may be measured simply by electrically connecting the measuring circuit to end 184 of feedthrough 180.
Conductor 134 is normally disposed in a circular xe2x80x9cring-likexe2x80x9d configuration on the lower surface of electrode 130 (as indicated in FIG. 2C). Further, some prior art pressure transducers include more than one conductor disposed on electrode 130 and a corresponding number of feedthroughs to electrically connect to the conductors. Such transducers provide at least two capacitors: a first capacitor formed by diaphragm 120 and one conductor on electrode 130 and a second capacitor formed by diaphragm 120 and another conductor on electrode 130. As is known, providing multiple capacitors in this fashion can be used to advantageously provide more accurate temperature compensation for the transducer.
In operation, transducer assembly 100 is normally used as an absolute pressure transducer. In this form, chamber 112 is normally first evacuated by applying a vacuum pump (not shown) to pressure tube 172. After chamber 112 has been evacuated, tube 172 is then sealed, or xe2x80x9cpinched offxe2x80x9d to maintain the vacuum in chamber 112. This creates a xe2x80x9creferencexe2x80x9d pressure in chamber 112. Although a vacuum is a convenient reference pressure, it is also known to use other pressures as the reference pressure. Since the pressure in chamber 112 is a known or reference pressure, the components used to construct chamber 112 (i.e., P_r body 160 and P_r cover 170) are referred to as P_r components (i.e., xe2x80x9creference pressurexe2x80x9d components). After the reference pressure has been established in chamber 112, pressure tube 142 is then connected to a source of fluid (not shown) to permit measurement of the pressure of that fluid. Coupling pressure tube 142 in this fashion delivers the fluid, the pressure of which is to be measured, to chamber 110 (and to the lower surface of diaphragm 120). Since the pressure in chamber 110 is unknown, or is to be measured, the components used to construct chamber 110 (i.e., P_x cover 140 and P_x body 150) are referred to as P_x components (i.e., xe2x80x9cunknown pressurexe2x80x9d components). The center of diaphragm 120 flexes up or down in response to the differential pressure between chambers 110 and 112. Transducer assembly 100 permits measurement of the amount of flexure of the diaphragm and thereby permits measurement of the pressure in chamber 110 relative to the known pressure in chamber 112.
Transducer assembly 100 can of course also be used as a differential pressure transducer. In this form, pressure tube 142 is connected to a first source of fluid (not shown) and pressure tube 172 is connected to a second source of fluid (not shown). Transducer assembly 100 then permits measurement of the difference between the pressures of the two fluids.
One problem with transducer assembly 100 relates to the zero pressure differential nominal spacing between conductor 134 and diaphragm 120. The reference distance between diaphragm 120 and conductor 134 for a particular reference pressure differential between chambers 110 and 112, for example, the zero pressure differential, may be referred to as the xe2x80x9cnominal distancexe2x80x9d or xe2x80x9cnominal gap.xe2x80x9d In operation of transducer assembly 100, diaphragm 120 of course flexes up and down, thereby changing the spacing between diaphragm 120 and conductor 134. However, for transducer assembly 100 to provide a consistently accurate pressure reading, it is important to provide a constant nominal distance between diaphragm 120 and conductor 134. So for a particular pressure differential, it is important to insure that the nominal distance between diaphragm 120 and conductor 134 is always the same. When manufacturing large numbers of transducer assemblies 100, it is important to consistently provide the same nominal distance between conductor 134 and diaphragm 120 in every unit. Further, in any one unit of transducer assembly 100, it is important to insure that the nominal distance remains constant and does not vary over time.
The nominal gap between the diaphragm and the electrode may be very small, e.g., in the range of 25 to 400 microns. The sensor may be made of a number of different materials, each of which may react differently in response to changes in temperature during manufacture or in use. Because the nominal gap and other tolerances are small, minor changes due to variations in temperature can have a significant effect on the nominal gap. For example, if the metallic housing expands in an axial direction (i.e., a direction perpendicular to the plane of the diaphragm) at a first rate, and the ceramic electrode 130 expands in the axial direction at a second rate, the nominal gap can change.
Prior art transducer assembly 100 includes a resilient element 192 for maintaining a constant nominal distance. Resilient element 192 is squeezed between P_r cover 170 and the top of electrode 130. Lower shoulder 164 of P_r body 160 supports shoulder 136 of electrode 130. Since P_r cover 170 is welded to P_r body 160, resilient element 192 provides a spring force that pushes down on electrode 130 and holds electrode 130 in a fixed position relative to P_r body 160. Resilient element 192 is often implemented using a xe2x80x9cwave washerxe2x80x9d (i.e., a metallic O-ring type washer that has been bent in one or more places in directions perpendicular to the plane of the ring). Resilient element 192 provides a relatively large spring force (e.g., on the order of one hundred pounds) so as to hold electrode 130 in a stable position.
Although transducer assembly 100 holds electrode 130 securely, the nominal distance between conductor 134 and diaphragm 120 can vary by small amounts over time in response to, for example, mechanical or thermal shock. As those skilled in the art will appreciate, elements that are held in place by compression, such as electrode 130, can exhibit small amounts of movement (sometimes referred to as xe2x80x9ccreepxe2x80x9d) over time. This creep can sometimes change the nominal distance and thereby adversely affect the accuracy of transducer assembly 100. Overpressure conditions can also cause unwanted movement of electrode 130. During normal operation of transducer assembly 100, diaphragm 120 will never contact electrode 130. However, large pressures in chamber 110 beyond the normal operating range of transducer assembly 100 (i.e., overpressure), can cause diaphragm 120 to contact electrode 130 and slightly compress resilient element 192. When the overpressure condition dissipates and diaphragm 120 returns to a normal operating position, resilient element 192 re-expands and reseats electrode 130. Sometimes the new position of electrode 130 will be slightly different than the original position prior to the overpressure condition. Such shifts in position can cause changes in the nominal distance and adversely affect the accuracy of transducer assembly 100.
The invention provides improved electrodes and mountings for electrodes in pressure transducers. Generally, the electrode and mounting designs improve the stability of the electrode.
In one aspect of the invention a pressure transducer includes a hub-and-spoke mounting in one chamber. The hub-and-spoke configuration extends in a plane parallel to the diaphragm. The reference electrode is suspended from the hub of the hub-and-spoke mounting and positioned proximate to the diaphragm. The hub-and-spoke mounting improves the stability of the reference electrode and the uniformity of the nominal gap. The hub, and thus the reference electrode, remains substantially motionless as the body of the pressure transducer is subjected to various forces. One advantage is that the hub-and-spoke mounting isolates the electrode from the forces applied to the body of the pressure transducer, such as fluctuations in the atmospheric pressure. Additionally, the mounting eliminates the need for a resilient element to position the reference electrode. The mounting also improves the pressure transducer""s response to overpressure conditions. The spokes may incorporate reentrant grooves to further improve the stability of the hub.
In another aspect of the present invention, a pressure transducer includes an improved reference electrode with a conductive support. The electrode is preferably entirely metallic. The electrode is rigidly affixed to an electrode mounting by a dielectric material. One advantage is that eliminating the ceramic from the electrode reduces the effect of stray capacitances. A second advantage is that the transducer will be more thermally stable because the electrode will have a thermal coefficient of expansion similar to that of the housing. Also, the dielectric joint will be more reliable than mechanical fasteners and maintain a fixed position of the electrode over time. Preferably, a spacer may set the nominal gap between the electrode and the diaphragm. In preferred embodiments, the electrode is a dual electrode, with a dielectric insulating a first electrode from a second electrode.
In another aspect of the invention, the transducer has an electrode with a disk portion mounted to a support post that provides mechanical support and is held to the housing with a joining material, such as glass, metallic solder or braze, ceramic, or glass-ceramic. The support post is preferably made of ceramic and may be a rod formed as a separate piece from the electrode, or a post portion that is formed as part of a unitary and monolithic disk and post. The joint between the post and the housing forms a compression type joint with very high strength. The material at this joint preferably has low stiffness, and one advantage, therefore, is that the joint is able to absorb some thermal expansion mismatch. The joint is able to relieve strain between the housing and the post at the joint in an elastic and predictable manner when the pressure transducer is subjected to temperature change. Also, another advantage is that the ceramic post, having a very high stiffness, further rejects thermally induced strains from reaching the disk. In yet another aspect of the invention, a ceramic electrode includes a groove in one face of the disk portion. The groove also relieves stress between the post portion and the disk portion caused by thermal expansion due to temperature variations.
In another aspect of the invention, a pressure transducer includes a member having a low thermal coefficient of expansion (TCE) relative to the housing. The housing is preferably metallic and supports a ceramic reference electrode by means of a ceramic rod joined to the electrode. The low TCE member is connected to the housing proximate to where the rod of the reference electrode is joined to the housing. The low TCE member helps prevent warping of the housing due to thermal effects when the pressure transducer is subjected to temperature variations during manufacture or use. The low TCE member may be rigidly connected in or opposite to an opening in the housing in which the rod is held. Alternatively, the housing may include a hub-and-spoke or similar mounting, and the low TCE member may be welded to the spokes on a side facing away from the electrode and the diaphragm.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein the invention is shown and described by way of illustration of embodiments of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims.